1. Field of the Invention
This invention relates generally to hydraulic control systems and more particularly to hydraulic control systems for utility vehicles.
2. Description of the Prior Art
To keep streets, parking lots, etc. free of debris and refuse motorized street sweepers are often used. Usually, the street sweeper apparatus is mounted on a truck's body, and includes a number of rotary sweeper brushes which engage the ground surface beneath the truck to propel the debris towards a conveyor system which deposits it into a hopper or other container. Some street sweepers include vacuum pickup heads as conveyors to draw the debris from the road surface and to deposit it in the hopper.
Hydraulics have been commonly used to control the position of the brooms, hopper, etc. Hydraulic actuators require a pressurized source of hydraulic fluid which, in turn, implies a hydraulic pump coupled to a power source. The most logical power source is the truck's internal combustion engine, although it is not always used for reasons to be discussed subsequently.
A major reason that a truck's internal combustion engine is often not used for powering a hydraulic pump is that the engine doesn't produce enough power at idling speeds to adequately pressurize the hydraulic fluid. One solution to this problem is to either manually or automatically increase the R.P.M. of the truck's engine until the hydraulic lines are sufficiently pressurized to power the actuators. This, however, tends to be wasteful and inefficient in that it results in high fuel consumption and in rapid wear of the engine's components.
Another problem with powering a hydraulic pump from the truck's engine is that the output of the pump will vary with the engine R.P.M. as the truck accelerates and decelerates. A prior art solution to this problem has been to automatically regulate the pump's output in response to operating conditions. For example, in U.S. Pat. No. 4,343,060, of Hildebrand et. al., means are used for electronically sensing the rotational speed of the sweeping components, and then feeding the speed signal into an electronic computing and control unit which sends electric power to an electric displacement control valve on a variable and reversible flow piston pump. Such control system, however, are complex and tend to raise the cost of the street sweeper dramatically.
Yet another problem with powering a hydraulic pump from a truck's engine is that the high starting load on the hydraulic pump will often kill the truck's engine. Prior art solutions to this problem include racing the truck's engine, which is very wasteful of fuel, or providing complex pump regulating devices such as that disclosed by Hildebrand et. al.
An alternative method for powering the hydraulic pump is to provide an auxiliary internal combustion engine mounted at a convenient location on the truck's body. This auxiliary engine would run at a relatively high, constant R.P.M. to power the hydraulic pump. Auxiliary engines, however, are expensive, and again tend to be wasteful of fuel since they are in continuous operation.